[libc++][ranges] Add ranges::in_in_result

Add `std::ranges::in_in_result`

Reviewed By: Quuxplusone, Mordante, #libc

Spies: ldionne, libcxx-commits, mgorny

Differential Revision: https://reviews.llvm.org/D116278

NOKEYCHECK=True
GitOrigin-RevId: d3729bb38475c5620cf4ec6c0dfd4613e5e64f8d
diff --git a/include/algorithm b/include/algorithm
index 932d17d..03b4faa 100644
--- a/include/algorithm
+++ b/include/algorithm
@@ -18,6 +18,11 @@
 namespace std
 {
 
+namespace ranges {
+  template <class I1, class I2>
+    struct in_in_result; // since C++20
+}
+
 template <class InputIterator, class Predicate>
     constexpr bool     // constexpr in C++20
     all_of(InputIterator first, InputIterator last, Predicate pred);
@@ -691,6 +696,7 @@
 #include <__algorithm/generate.h>
 #include <__algorithm/generate_n.h>
 #include <__algorithm/half_positive.h>
+#include <__algorithm/in_in_result.h>
 #include <__algorithm/in_out_result.h>
 #include <__algorithm/includes.h>
 #include <__algorithm/inplace_merge.h>